Grasslands are found in Illinois where climate and soils produce trees in
others parts of the world. The difference is fire.

Fires occur spontaneously in nature, but archeological evidence confirms that
the early humans of Illinois also deliberately burned their range. Fire rendered
the landscape more hospitable--by driving game to slaughter, by clearing brush
from fields, and by killing off woody plants that competed with the (mainly)
annual plants that supplied the seeds and tubers upon which advanced hunter-gatherers
relied. Many of the plants that were nurtured by gatherers and later by the
first farmers are quick to exploit disturbed areas. Burning was an easy way
to see that the landscape stayed disturbed, to the benefit of both annual plants
and plant-gatherers.

Some researchers have speculated that, were it not for the fires set by Native
Americans, forest would have covered as much as 20% more of the Illinois landscape
at the time the French explorers arrived. According to this interpretation,
prairies are cultural as much as natural features. The mixed-grassland-forest
of presettlement Illinois was a result of what geographer Carl Sauer called
"the appropriation of habitat by habit."

What human hands did to help create the prairies, they can also undo. For
instance, by putting away their plows and putting out fires, humans virtually
ensure that trees will take over an abandoned farm field. Hot, dry climates
favor grasses over trees. There was a resurgence of hot summers in the 1980s,
but even an extended warming trend favoring them would not necessarily cause
grasslands to increase in Illinois without periodic cultivation or fires to
suppress competing trees--although a warming trend might at least slow the advance
of woody plants into extant prairies. In the absence of periodic cultivation
or fires to suppress them, trees, rather than prairie, will take over an abandoned
farm field.

As a result, hardwood forests have spontaneously, if modestly, increased their
extent in Illinois in recent years. From 1820 to 1980 an estimated 2.64% of
Illinois land (mainly abandoned farm fields) reverted to forest. With burning
stopped, only the plow has retarded the natural reforestation of Illinois.